Intravascular ultrasound is a rapidly evolving imaging technique most commonly employed in coronary and iliofemoral arteries. The technique has the potential to facilitate the study of aneurysm progression, atherosclerosis or dissection and to outline the effect of endovascular intervention in more detail than angiography.
The presently used intravascular ultrasound systems fall into two categories: stationary electronic systems and mechanically driven rotating transducer systems. In both systems, an acoustic element or transducer is used to transmit a signal, which impinges upon, and reflects from, surfaces of different acoustic densities, which the signal encounters. An acoustic transducer receives the reflected wave. These data are sent to a processing system via an electrical cable where they are manipulated and displayed as an image. The systems are mounted to catheters, or axially elongate structures, which are routed through body lumens such as arteries to reach the site of imaging.
The non-rotating or stationary catheter of the stationary electronic system houses an array of small acoustic elements, which are positioned cylindrically at the catheter tip. After positioning the catheter in a vessel, body lumen or cavity, subgroups of acoustic elements together generate an echo image. The spacing between the acoustic elements in the transducer array creates areas where the acoustic signal is neither transmitted nor received. When the data is processed, gaps of missing information occur, resulting in a poor quality image. The advantage of the stationary electronic system is that the catheter is very flexible and a central lumen is available for guidewire insertion. No distortion of the image, due to inhomogeneous mechanical rotation, is present. The stationary catheters are reliable and inexpensive but produce a poor quality image.
The mechanical intravascular ultrasound-imaging catheter comprises a mechanically rotating catheter shaft with a single ultrasound transducer. Either the acoustic element rotates or the acoustic element is stationary and a mirror rotates. In this manner, the acoustic signal is transmitted and received in a continuous 360-degree sweep. There are no gaps in the data and a higher quality image results. Realizing a driving mechanism while keeping the catheter fully flexible and steerable as well as miniature are challenging problems. Distortion of the image due to an unequal rotation of the element or mirror at the catheter tip is a limitation of these systems. Advantages of the mechanical probes include high-resolution imaging and absence of near field artifact. The mechanically rotating devices produce an acceptable image but are unreliable and expensive.
Both stationary electronic systems and mechanical systems typically operate with acoustic frequencies from 10 to 30 MHz.
Medical interventions are often performed using endovascular techniques. These interventions include balloon dilatation, atherectomy, stent or device placement and removal, drug delivery, thrombolytic therapy, thrombectomy, vessel irradiation, embolic device delivery and thermal therapy by radio waves or microwaves. Guidance of these endovascular interventions is preferably accomplished using intravascular ultrasound imaging.